This invention relates to the treatment of toxic and/or environmentally hazardous or harmful materials and more especially to the treatment of aqueous solutions of hazardous waste arising from various chemical processes.
The chemical processing industry in general generates vast quantities of by-products and waste materials many of which represent environmental hazards and which must be neutralised or destroyed as an essential part of their ultimate disposal. The oil and gas processing industries, for example, invest heavily in plant and equipment designed specifically to prevent or minimise the release of harmful largely organic materials into the environment. The microelectronics and semi-conductor device manufacturing industries, for example, make similar investments in order to scrub or otherwise treat exhaust gas streams containing generally inorganic materials from chemical processing units prior to the release of those exhaust gases into the atmosphere. Compounds containing heavy metals and halogen-, sulphur-, phosphorus- and nitrogen-containing compounds can be especially toxic and their removal is the subject of a considerable amount of technical research and of much environmental protection legislation.
Many of the procedures utilised in the microelectronics and semi-conductor device manufacturing industries use fluorine-containing compounds. Unused fluorine-containing compounds and fluorine-containing products and by-products are generally discharged from the process or from a subsequent exhaust treatment, for example scrubbers and other adsorption systems, as an aqueous waste stream. This aqueous waste stream will generally contain fluorine in the form of HF. However, cationic species such as NH4+ may also be present, in addition to fluoride in the form of bifiuoride ions, HF2−.
Current practise in such manufacturing and disposal facilities involves treating the aqueous fluoride stream with magnesium or calcium salts in order to precipitate the sparingly soluble MgF2 and CaF2, usually the latter. The solid material can then be compacted and dried for ease of transport for disposal or further use. However, the water stream will still have a fluoride content of some 20 to 30 ppm which continues to present a disposal problem with discharge limits of 3 ppm being more generally imposed.
Further, some legislative areas prohibit the dilution of certain waste streams (for example, aqueous fluoride with an aqueous stream containing no fluoride) for disposal purposes and in other areas allowable discharge is based on the quantity of discharged species and not their concentrations. The safe disposal of hazardous and harmful materials therefore presents ever increasing problems.
A further difficulty is that effluent from recently developed procedures which use a mixture of ammonium bifiuoride and HF cannot be accommodated in a calcium precipitation installation because CaF2 will not form at pH below 12 and such high pH values favour the dissociation of ammonium species to produce ammonia gas. Additional facilities are then required to remove and separate NH4+ and F− before they can be treated to form solid waste. This can be achieved using traditional ion exchange techniques but the plant will be large in that it will require separate beds for the two ion exchange species and duplex systems with further chemical feeds for their periodic regeneration.
A recently developed system that is capable of removing both anionic and cationic species from aqueous solutions without requiring further chemical additives makes use of a technique known as electrochemical deionisation which involves ion exchange and electrolytic separation technologies. In this system cations or anions of interest are adsorbed from dilute aqueous solution onto an ion exchange medium, transported through that medium by an applied electric field and continuously eluted as a concentrated stream. Such a procedure is described in EP 0680932B. There are many examples within the existing literature of electrochemical cells that combine adsorption and ion separation and EP 0680932B illustrates one such ion removal/separation/concentration process. Other approaches will be known to those skilled in the art and can also be used. Such systems have been applied with some success on a continuous basis to minimise water consumption and to concentrate anions or cations for ease of subsequent handling in our copending Patent Application No. GB 0300793.7. That application demonstrates especially that fluoride ions can be concentrated and removed from a closed loop circulation system.
Further work has now revealed that the techniques and procedures described in GB 0300793.7 may be applied successfully to the problems described above. In this respect the structure of the electrochemical deionisation cell described and illustrated in GB 0300793.7 has been modified to permit the simultaneous removal of both anions and cations.